1. Technical Field
The present invention relates to a light source apparatus, a driving method of a discharge lamp, and a projector.
2. Related Art
Discharge lamps such as a high-pressure mercury lamp and a metal halide lamp are currently employed as a light source of a projector.
Such discharge lamps are driven by supplying, for example, a high-frequency alternating current as the driving current. Such a driving method provides stabilized discharge performance and prevents blackening or devitrification of the discharge lamp main portion thereby suppressing degradation of the life span of the discharge lamp, for example as described in JP-A-2007-115534.
However, while the discharge lamp is turned on arc discharge takes place between a pair of electrodes, and the electrodes melt because of a high temperature and thus the clearance between the electrodes become wider. In the case where the discharge lamp is employed for a projector, it is preferable to maintain a narrow clearance between the electrodes so as to illuminate a smaller region, in order to achieve a higher utilization efficiency of the light, and the increase of the clearance between the electrodes is not desirable since it leads to lower utilization efficiency of the light. The increase of the clearance between the electrodes also incurs fluctuation of the impedance therebetween, and therefore although the discharge lamp provides high efficiency during an initial period of use, impedance mismatch occurs with the lapse of time, which leads to an increase in reactive power and degradation of efficiency.
On the other hand, a low-frequency alternating current that presents a rectangular waveform (square AC) is also employed as the driving current. With such a driving method, a projection is formed on each of the pair of electrodes with the use of the discharge lamp, which contributes to maintain the narrow clearance between the electrodes.
In this case, however, the discharge lamp main portion is prone to suffer blackening and devitrification, resulting in reduced life span of the discharge lamp.
Accordingly, the present inventor has developed a driving method including modulating amplitude of alternating current of a frequency not lower than 1 kHz and not higher than 10 GHz, and supplying the amplitude-modulated alternating current to the discharge lamp as driving current, thereby turning on the discharge lamp.
The driving method suppresses the blackening of the discharge lamp, and allows the clearance between the electrodes to be maintained constant while driving the discharge lamp, in the case where the luminance of the discharge lamp remains unchanged.
However, in the case where the discharge lamp is of a type that allows the luminance to be controlled, the clearance between the electrodes may become narrower or wider depending on the luminance.